Hydrogen gas sensor

ABSTRACT

A hydrogen gas sensor capable of accurately measuring hydrogen concentration of a measurement gas atmosphere in the presence of a variety of interfering gasses such as H 2 O and CO. In the hydrogen gas sensor, the flow sectional area of a diffusion-rate limiting portion  6  is rendered small; the electrode surfaces of first and second electrodes  3  and  4  are rendered large; and/or a solution containing a polymer electrolyte which may be identical to that of a proton-conductive layer  2  is applied onto the surfaces of the first and second electrodes  3  and  4  to thereby form a layer containing the polymer electrolyte. Thus, the rate of conduction of protons from the first electrode  3  to the second electrode  4  becomes greater than the rate at which protons are derived from hydrogen which is introduced onto the first electrode  3  via the diffusion-rate limiting portion  6.

RELATED APPLICATION

This is a continuation of application Ser. No. 09/716,225 filed Nov. 21,2000, issue as U.S. Pat. No. 6,652,723; the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrogen gas sensor, and moreparticularly, to a hydrogen gas sensor suitable for measuring thehydrogen concentration of a fuel gas used for fuel cells.

2. Description of the Related Art

In view of the issue of global-scale environmental deterioration, fuelcells, which are clean and efficient power sources, have recently becomethe subject of active studies. Among fuel cells, a polymer electrolytefuel cell (PEFC) is expected to be suitable for vehicle use due to itsadvantages, including low operation temperature and high output density.In this case, a reformed gas obtained from methanol or the like isadvantageously used as a fuel gas. Further, in order to improveefficiency and other parameters of performance, a gas sensor capable ofdirectly measuring hydrogen concentration of the reformed gas is needed.

Since such a hydrogen gas sensor is used in a hydrogen-rich atmosphere,the operation temperature of the gas sensor must be low (about 100° C.or less). Such a low-operation-temperature-type sensor is disclosed inJapanese Patent Publication (kokoku) No. 7-31153. In the sensor, aworking electrode, a counter electrode, and a reference electrode aredisposed on an insulating substrate, and the three electrodes areintegrally covered with a gas-permeable, proton-conductive film; morespecifically, “NAFION®” (trademark, product of Dupont), which is a typeof fluororesin. NAFION® is a proton-conductive material capable ofoperating at low temperature and is used at portions of polymerelectrolyte fuel cells.

The present Inventors found that when NAFION® is used as aproton-conductive layer as in the gas sensor disclosed in JapanesePatent Publication No. 7-31153, the sensor output varies depending onthe H₂O concentration partial pressure of a gas under measurement(hereinafter referred to as a measurement gas atmosphere), so thataccurate measurement becomes difficult. Further, the present Inventorsfound that the above phenomena occurs because protons pass throughNAFION® together with H₂O molecules, and therefore, the protonconductivity varies with the H₂O concentration of the measurement gasatmosphere. That is, when the proton-conductive layer is formed ofNAFION®, the sensor output depends on the H₂O concentration of themeasurement gas atmosphere, so that the sensor output decreases greatly,especially when the H₂O concentration is low.

The present Inventors further found that although porous Pt electrodes(catalysts) are generally known to exhibit high activity at lowtemperature (porous Pt electrodes are used, for example, in fuel cells),when such a Pt electrode is exposed to an atmosphere having a high COconcentration, CO is adsorbed on the Pt electrode, or the Pt electrodeis CO-poisoned, so that the sensor output is greatly decreased.

Since many fuel cells use pressurized fuel gas in order to improve powergeneration efficiency, sensors used in the fuel gas are required to havea small pressure dependency. However, in the sensor described in theabove-mentioned Japanese Patent Publication No. 7-31153, gas undermeasurement is diffused to the working electrode via the gas-permeable,proton-conductive film, so that the sensor exhibits a great pressuredependency, depending on the structure of the proton-conductive filmitself, and therefore high measurement accuracy cannot be obtained.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a hydrogengas sensor capable of accurately measuring hydrogen concentration in thepresence of a variety of interfering gasses.

In the hydrogen gas sensor of the present invention, the rate ofconduction of protons from a first electrode to a second electrode isrendered greater than the rate at which protons are derived fromhydrogen introduced onto the first electrode via a diffusion-ratelimiting portion.

That is, because the rate of conduction of protons from the firstelectrode to the second electrode is sufficiently greater than the rateat which protons are derived from hydrogen introduced from themeasurement gas atmosphere onto the first electrode via thediffusion-rate limiting portion, the sensor can accurately measurehydrogen concentration without causing a great decrease in sensoroutput. That is so even when the measurement gas atmosphere has a lowH₂O concentration or a high CO concentration.

The present invention is applicable to both a hydrogen gas sensor nothaving a reference electrode and to a hydrogen gas sensor having areference electrode. In the latter gas sensor, the voltage appliedbetween the first and second electrodes can be variably controlled suchthat a constant voltage is produced between the first electrode and thereference electrode, or such that the hydrogen concentration on thefirst electrode becomes constant. Therefore, for any given hydrogenconcentration an optimal voltage can be applied between the first andsecond electrodes, so that a more accurate measurement of hydrogenconcentration can be obtained within a wide range of concentration.

The hydrogen gas sensor according to the present invention isadvantageously used for measuring an atmosphere in which hydrogen H₂O,and other components coexist, especially for measuring the hydrogenconcentration of a fuel gas for polymer electrolyte fuel cells.

In a preferred mode of the present invention, the diffusion-ratelimiting portion preferably has a relatively high gas-diffusionresistance, so as to render the proton-conducting performance excessive.In this case, the rate of conduction of protons through theproton-conductive layer becomes greater than the rate at which protonsare derived from hydrogen introduced onto the first electrode. Thegas-diffusion resistance of the diffusion-rate limiting portion isincreased, for example, by increasing the length (thickness) of thediffusion-rate limiting portion in the gas diffusion direction or bydecreasing the cross sectional area perpendicular to the gas diffusiondirection (hereinafter referred to as a “flow sectional area”).Alternatively, when the diffusion-rate limiting portion is formed of aporous material, the gas-diffusion resistance of the diffusion-ratelimiting portion is increased by decreasing the porosity (pore diameter,apparent porosity, etc.) of the porous material.

The gas-diffusion resistance of the diffusion-rate limiting portion ispreferably set as follows in order to render the rate of conduction ofprotons from the first electrode to the second electrode greater thanthe rate at which protons derived from hydrogen are introduced onto thefirst electrode via the diffusion-rate limiting portion.

(1) Proton conduction condition A: A proton-conducting rate under severeconditions is measured. That is, a current (a) flowing between the firstand second electrodes is measured upon applying a sufficiently highvoltage between the first and second electrodes in a state in which thegas-diffusion resistance of the diffusion-rate limiting portion isrendered sufficiently small (e.g., about 0.9 mA/mm² or more of the firstelectrode (3), with current conversion, at H₂=40%) in order to introducea sufficiently large amount of hydrogen onto the first electrode, butunder the severest conditions for proton conduction; e.g., conditionssuch that the measurement gas atmosphere has a very low H₂Oconcentration (specifically, 10% or less at 80° C.) or a very high COconcentration (specifically, 1000 ppm or greater). Although theabove-described current (a) need not be a saturation current, theapplied voltage (specifically, 50 mV or higher) is preferably equal toor higher than the voltage applied in the case of condition B describedbelow.

(2) Proton conduction condition B: Next, a proton-conducting rate underfavorable conditions is measured. That is, a saturation current (b)flowing between the first and second electrodes is measured uponapplication of a sufficiently high voltage between the first and secondelectrodes in a state in which the gas-diffusion resistance of thediffusion-rate limiting portion is rendered larger (e.g., less thanabout 0.9 mA/mm² of the first electrode (3), with current conversion, atH₂=40%) in order to sufficiently reduce the amount of hydrogenintroduced onto the first electrode, but under favorable conditions forproton conduction, e.g., conditions such that the measurement gasatmosphere has a sufficiently high H₂O concentration (specifically, 15%or greater, more preferably 20or greater, at 80° C.) or a sufficientlylow CO concentration (specifically, 800 ppm or less). The sufficientlyhigh voltage for producing a saturation current (b) is 300 mV or more atH₂=40%, and varies according to the H₂ concentration as shown in FIG. 2.In the case of H₂=10%, it is about 100 mV or more.

(3) Setting of gas-diffusion resistance: When the gas-diffusionresistance of the diffusion-rate limiting portion is set to asufficiently high value under condition B, current (a)>saturationcurrent (b). Thus, the hydrogen gas sensor is configured such thatproton-conducting rate (current value) under the severest conditions forproton conduction>proton-conducting rate under favorable conditions forproton conduction. In this hydrogen sensor, the proton-conducting rateis always greater than the proton-generation rate corresponding to therate at which hydrogen is introduced onto the first electrode (or thelargest proton-generation rate corresponding to the largest rate atwhich hydrogen is introduced onto the first electrode).

In yet another preferred mode of the present invention, current (c)flowing between the first and second electrodes is measured under severeconditions for proton conduction; current (d) flowing between the firstand second electrodes is measured under favorable conditions for protonconduction; and the gas-diffusion resistance of the diffusion-ratelimiting portion is set such that the ratio of current (d) to current(c) (=current (d)/current(c)) or its reciprocal (=current(c)/current(d)) approaches 1. As a result, the H₂O-concentrationdependency and CO-concentration dependency of the current flowingthrough the first and second electrodes decrease. Preferably, thegas-diffusion resistance of the diffusion-rate limiting portion and/orthe area of the first or second electrode is properly set, or apredetermined polymer electrolyte solution is applied to the interfaceof the first or second electrode which is in contact with theproton-conductive layer, such that the ratio (saturation current flowingbetween the first and second electrodes at H₂O=30%)/(saturation currentflowing between the first and second electrodes at H₂O=10%) falls withinthe range of 1 to 1.5, preferably 1 to 1.15, more preferably, 1 to 1.1,most preferably, 1 to 1.05. Further, preferably, the gas-diffusionresistance of the diffusion-rate limiting portion and/or the area of thefirst or second electrode is appropriately set, or a polymer electrolytesolution is applied to the interface of the first or second electrodewhich is in contact with the proton-conductive layer, such that theratio (saturation current flowing between the first and secondelectrodes at CO=1000 ppm)/(saturation current flowing between the firstand second electrodes at CO=0 ppm) falls within the range of 0.9 to 1(the reciprocal of the rate falls within the range of 1 to 1.1), morepreferably, 0.95 to 1 (the reciprocal of the rate falls within the rangeof 1 to 1.05). Thus, a layer containing a polymer electrolyte is formedat the interface.

In a preferred mode of the present invention, the first and secondelectrodes are formed in an opposed manner to sandwich theproton-conductive layer. This structure reduces the resistance betweenthe first and second electrodes to thereby improve the proton-conductingperformance of the proton-conductive layer. However, when thegas-diffusion resistance of the diffusion-rate limiting portion isexcessively high, the sensitivity of the hydrogen gas sensor is lowered.Therefore, the area of at least one of the first and second electrodesis preferably increased when the sensor must have a relatively highsensitivity. Further, the first and second electrodes may be formed onthe same plane of the proton-conductive layer, if a sufficient degree ofsensitivity can be achieved.

In a preferred mode of the present invention, a solution containing apolymer electrolyte identical to that of the proton-conductive layer isapplied to the side of each electrode in contact with theproton-conductive layer (the interface between each electrode and theproton-conductive layer). This increases the contact area between theproton-conductive layer and catalytic components carried on theelectrode, so that the proton-conducting performance is furtherincreased. Further, the proton-conducting performance may be enhanced bydecreasing the thickness of the proton-conductive layer.

In a preferred mode of the present invention, the proton-conductivelayer is a polymer electrolytic proton-conductive layer whichsufficiently operates at a relatively low temperature, for example, attemperatures not greater than 150° C., preferably, at temperatures notgreater than 130° C., more preferably, at around 80° C.; e.g., aproton-conductive layer formed of a resin-based solid polymerelectrolyte.

In a preferred mode of the present invention, a proton-conductive layeris formed of one or more types of fluororesins, more preferably of“NAFION®” (trademark, product of Dupont).

In a preferred mode of the present invention, each electrode is a porouselectrode formed of carbon or another suitable material, and carries acatalyst such as Pt on the side of the electrode in contact with theproton-conductive layer.

In the preferred mode of the present invention, the proton-conductivelayer, the respective electrodes, and the diffusion-rate limitingportion are supported on a support to thereby constitute an integratedhydrogen gas sensor. The support is formed of an inorganic insulatingmaterial such as an alumina ceramic or an organic insulating materialsuch as resin. Further, the diffusion-rate limiting portion ispreferably formed of a porous alumina ceramic or a like material havinggas permeability, or alternatively may be formed of small holes eachhaving a small flow sectional area, such as one or more through-holeseach having a very small opening diameter, which are formed at a portionof the support formed of a dense body. Such fine through-holes can beformed by use of, for example, laser machining or ultrasonic machining.In the case of laser machining, the opening diameter may be adjusted bycontrolling the irradiation diameter, output power, time, etc., of alaser beam. The average pore diameter of the porous material and theopening diameter of the through-holes are preferably not less than 1 μm.In this case, since gas diffusion proceeds outside the region of Knudsendiffusion, the pressure dependency of the sensor can be lowered.

The hydrogen gas sensor of the present invention can be fabricated byphysically sandwiching the proton-conductive layer and the respectiveelectrodes between two supports such that the respective electrodescontact the proton-conductive layer. Alternatively, the respectiveelectrodes may be bonded to the proton-conductive layer by hot pressing.

In the preferred mode of the present invention, a hydrogen gas sensornot having a reference electrode has a support for supporting theproton-conductive layer, the first electrode, the second electrode andthe diffusion-rate limiting portion, and a hydrogen gas sensor providedwith a reference electrode has a support for supporting theproton-conductive layer, the first electrode, the second electrode, thereference electrode and the diffusion-rate limiting portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a main portion of a hydrogen gas sensoraccording to the present invention;

FIG. 2 is a graph of the results of measurement 1, showing an appliedvoltage-current characteristic for each of various hydrogenconcentrations;

FIG. 3 is a graph of the results of measurement 2-1, showing therelationship between flow sectional area of the diffusion-rate limitingportion and current (sensitivity) at H₂=40%;

FIG. 4 is a graph of the results of measurement 2-2, showing thedependency of current (sensitivity) on flow sectional area(gas-diffusion resistance) of the diffusion-rate limiting portion andH₂O concentration;

FIG. 5 is a graph of the results of measurement 2-3, showing thedependency of current (sensitivity) on flow sectional area(gas-diffusion resistance) of the diffusion-rate limiting portion and COconcentration;

FIG. 6 is a graph of the results of measurement 3, showing thedependency of current (sensitivity) on electrode area and H₂Oconcentration;

FIG. 7 is a graph of the results of measurement 4, showing therelationship between H₂O concentration and resistance between the firstand second electrodes for the case where a solution of a polymerelectrolyte was applied, and for the case where the solution was notapplied;

FIG. 8 is a sectional view of a main portion of another hydrogen gassensor of the present invention;

FIG. 9 is a graph of the results of measurement 5, showing therelationship between hydrogen concentration and current (current flowingbetween the first and second electrodes) at each of different H₂Oconcentrations;

FIG. 10 is a graph of the results of measurement 5, showing therelationship between hydrogen concentration and applied voltage at eachof different H₂O concentrations; and

FIG. 11 is a graph in relation to measurement 6, showing therelationship between gas pressure and current (current flowing betweenthe first and second electrodes) measured for each of different porediameters of the diffusion-rate limiting portion.

Reference numerals are used in the drawings to identify the following:

1 a, 1 b: upper and lower supports

2: proton-conductive layer

3: first electrode

4: second electrode

5: reference electrode

6: diffusion-rate limiting portion

7: power source

8: ammeter

9: power source

10: voltmeter

11: aperture

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described with reference tothe drawings in order to clarify the above-described preferred mode ofthe present invention. However, the present invention should not beconstrued as being limited thereto.

First, the structure of a hydrogen gas sensor according to the presentinvention will be described. FIG. 1 is a sectional view of a mainportion of the hydrogen gas sensor. As shown in FIG. 1, in the hydrogengas sensor, a first electrode 3 and a second electrode 4 are disposed onopposite surfaces, respectively, of a proton-conductive layer 2 suchthat the first electrode 3 and the second electrode 4 sandwich theproton-conductive layer 2. Further, an upper support 1 a and a lowersupport 1 b sandwich the first electrode 3 and the second electrode 4. Adiffusion-rate limiting portion 6 is formed in the upper support 1 alocated above the first electrode 3. The diffusion-rate limiting portion6 located between the first electrode 3 and a measurement gasatmosphere, and the second electrode 4 contacts the measurement gasatmosphere via an aperture 11 formed in the lower support 1 b.

A series circuit comprising a power source 7 and an ammeter 8 isconnected between the first and second electrodes 3 and 4 via leadportions, so that a voltage is applied between the first and secondelectrodes 3 and 4, and current flowing between the first and secondelectrodes 3 and 4 can be measured.

Next, the measurement principle of the hydrogen gas sensor will bedescribed with reference to FIG. 1.

(1) Hydrogen having reached the first electrode 3 via thegas-diffusion-rate limiting portion 6 is dissociated into protons byvirtue of the catalytic action of a catalytic component such as Ptcarried on the first electrode 3 and the voltage applied between thefirst electrode 3 and the second electrode 4.

(2) The protons thus generated are conducted to the second electrode 4via the proton-conductive layer 2 and are converted to hydrogen (gas) onthe second electrode 4, which hydrogen diffuses into the measurement gasatmosphere via the aperture 11. When the applied voltage is sufficientlyhigh such that a saturation current flows between the first electrode 3and the second electrode 4, the current flowing between the firstelectrode 3 and the second electrode 4 varies in proportion to thehydrogen concentration. Therefore, the hydrogen concentration can bemeasured by detecting the saturation current using ammeter 8.

Measurement of hydrogen concentration was performed using theabove-described hydrogen gas sensor (see FIG. 1). In the hydrogen gassensor, the proton-conductive layer was formed of NAFION®; the first andsecond electrodes were porous carbon electrodes carrying a catalyst suchas Pt on the side in contact with the proton-conductive layer; thesupport was formed of dense alumina ceramic; and the diffusion-ratelimiting portion was formed of porous alumina ceramic.

Measurement 1

For each of various hydrogen concentrations (hydrogen concentrations ofa gas under measurement), current flowing between the first and secondelectrodes 3 and 4 was measured, while the voltage applied between thefirst and second electrodes 3 and 4 was varied. Measurement conditionswere as follows.

Measurement Conditions

Gas composition: H₂ (0-40%), CO₂ (15%), H₂O (25%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 1/min; and Applied voltage: 0-800mV.

Next, results of the measurement will be described. FIG. 2 is a graphshowing an applied voltage/current characteristic for each of varioushydrogen concentrations. As shown in FIG. 2, a saturation current flowswhen the applied voltage exceeds about 400 mV, and the magnitude of thesaturation current varies in proportion to the hydrogen concentration ofthe measurement gas. Therefore, the hydrogen concentration can bemeasured using the hydrogen gas sensor.

Embodiment 1, Measurement 2

Next, with reference to the following measurement results, an examplemethod of rendering the rate of conduction of protons from the firstelectrode to the second electrode greater than the rate at whichhydrogen is introduced onto the first electrode via the diffusion-ratelimiting portion is described, as well as the effect thereof.

Measurement 2-1

The structure of hydrogen gas sensors used in the present measurementwill be described. Hydrogen gas sensors having the above-describedstructure (see FIG. 1) but differing from one another in terms ofgas-diffusion resistance of the diffusion-rate limiting portion werefabricated. Specifically, the cross sectional area of the diffusion-ratelimiting portion as measured perpendicular to the gas diffusiondirection was changed among the hydrogen gas sensors. The same voltagewas applied between the first and second electrodes 3 and 4 of eachhydrogen gas sensor, and the current flowing between the first andsecond electrodes 3 and 4 was measured. The measurement conditions wereas follows.

Gas composition: H₂ (40%), CO₂ (15%), H₂O (20%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 1/min; and Applied voltage: 800mV.

Next, the results of the measurement will be described. FIG. 3 is agraph, showing the relationship between the flow sectional area of thediffusion-rate limiting portion and current (sensitivity) at H₂=40%. Asshown in FIG. 3, the current decreases with the flow sectional area ofthe diffusion-rate limiting portion, and therefore, the amount ofhydrogen introduced onto the first electrode can be reduced byincreasing in the gas-diffusion resistance of the diffusion-ratelimiting portion.

Measurement 2-2

At each of various H₂O concentrations, a measurement similar tomeasurement 2-1 was performed using the gas sensors of measurement 2-1.The measurement conditions were as follows.

Gas composition: H₂ (40%), CO₂ (15%), H₂O (10, 20, 30%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 1/min; and Applied voltage: 800mV.

FIG. 4 is a graph describing the results of measurement 2-2 and showingthe dependency of current (sensitivity) on the flow sectional area(gas-diffusion resistance) of the diffusion-rate limiting portion andH₂O concentration. Since the absolute value of current (sensitivity)varies with the flow sectional area, each of currents (sensitivities) atH₂O=20% and 30% is represented as a ratio to current (sensitivity) atH₂O=10%. The absolute values of the respective currents are shown inTable 1.

TABLE 1 Current (Absolute value) (mA) Sectional area of Diffusion-ratelimiting portion H₂O concentration 4.3 mm² 2.7 mm² 1.4 mm² 10% (c) 7.316(a) 2.097 0.665 20% 8.296 2.173 0.688 30% (d) 8.671 2.247 0.693 (b) (a)Current under proton conduction condition A; i.e., low gas-diffusionresistance of the diffusion-rate limiting portion, and severe conditionsfor proton conduction; (b) Current under proton conduction condition B;i.e., high gas-diffusion resistance of the diffusion-rate limitingportion, and favorable conditions for proton conduction; (c) Severeconditions for proton conduction; and (d) Favorable conditions forproton conduction.

As shown in FIG. 4, the H₂O-concentration dependency decreases with theflow sectional area of the diffusion-rate limiting portion. Therefore,when the gas-diffusion resistance of the diffusion-rate limiting portionis increased to thereby render the proton-conductive performanceexcessive relative to the proton generation rate, the H₂O-concentrationdependency of the sensitivity or detection output of the sensor can bereduced. Further, Table 1 demonstrates that since current (a)>current(b), the rate of conduction of protons from the first electrode to thesecond electrode is greater than the rate at which protons are derivedfrom hydrogen which is introduced onto the first electrode via thediffusion-rate limiting portion. Further, as shown in FIG. 4, forH₂O=30%, the current ratio (current (d)/current (c)) was 1.185 when thesectional area of the diffusion-rate limiting portion was 4.3 mm², 1.071when the sectional area was 2.7 mm², and 1.041 when the sectional areawas 1.4 mm².

Measurement 2-3

The CO-concentration dependency of current (sensitivity) wasinvestigated using the gas sensors of measurement 2-2. Specifically, ateach of various CO concentrations, a measurement similar to measurement2-2 was performed. The measurement conditions were as follows.

Gas composition: H₂ (40%), CO₂ (15%), H₂O (25%), CO (0, 1000 ppm), N₂(bal.); Gas temperature: 80° C.; Gas flow rate: 4 1/min; and Appliedvoltage: 800 mV.

FIG. 5 is a graph describing the results of measurement 2-3 and showingthe dependency of current (sensitivity) on the flow sectional area(gas-diffusion resistance) of the diffusion-rate limiting portion and COconcentration. Since the absolute value of current (sensitivity) varieswith the flow sectional area, current (sensitivity) at CO=1000 ppm isrepresented as a ratio to current (sensitivity) at CO=0 ppm.

As shown in FIG. 5, the CO-concentration dependency decreases with adecrease in the flow sectional area of the diffusion-rate limitingportion. Therefore, when the gas-diffusion resistance of thediffusion-rate limiting portion is increased to thereby render theproton-conductive performance excessive relative to the protongeneration rate, the influence of CO-poisoning of Pt serving anelectrode catalyst can be reduced. Further, as shown in FIG. 5, thecurrent ratio (current (c: CO=1000 ppm)/current (d: CO=0 ppm)) was0.8785 (the reciprocal was 1.138) when the sectional area of thediffusion-rate limiting portion was 4.3 mm², 0.9813 (the reciprocal was1.019) when the sectional area was 2.7 and 0.99996 (the reciprocal was1.00003) when the sectional area was 1.4 mm².

As described above, since the gas-diffusion resistance of thediffusion-rate limiting portion is increased to thereby render theproton-conductive performance excessive relative to the protongeneration rate, the influence, for example, of H₂O and CO present in ameasurement gas atmosphere can be reduced, thereby enabling a moreaccurate measurement of hydrogen concentration. The above-describedmethod of increasing the gas-diffusion resistance is a mere example, andthe gas-diffusion resistance may be increased by increasing the lengthof the diffusion-rate limiting portion with respect to the gas diffusiondirection or by decreasing the pore diameter or porosity (open-poreratio) of the porous material that constitutes the diffusion-ratelimiting portion.

Embodiment 2, Measurement 3

Next, an example method in which the proton-conducting performanceitself is improved in order to render the rate of conduction of protonsfrom the first electrode to the second electrode greater than the rateat which hydrogen gas is introduced onto the first electrode isdescribed.

The structure of hydrogen gas sensors used in the present measurementwill next be described. Hydrogen gas sensors having the above-describedstructure (see FIG. 1) but differing from one another in terms of theareas of the first and second electrodes were fabricated. A measurementsimilar to measurement 2-2 was performed using the thus-fabricatedhydrogen gas sensors.

FIG. 6 is a graph for describing the results of measurement 3 andshowing the dependency of current (sensitivity) on electrode area andH₂O concentration. For each electrode area, each of currents(sensitivities) at H₂O=20% and 30% is represented as a ratio to current(sensitivity) at H₂O=10%.

As shown in FIG. 6, when the electrode area is about doubled, theproton-conducting performance is improved, so that the H₂O-concentrationdependency of the gas sensor sensitivity can be greatly reduced.

Embodiment 3, Measurement 4

Next, an example method in which a solution containing a polymerelectrolyte that constitutes the proton-conductive layer is applied tothe sides of the first or second electrode which are in contact with theproton-conductive layer (at the interfaces) is described, in order toimprove the proton-conducting performance itself, whereby the rate ofconduction of protons from the first electrode to the second electrodeis rendered greater than the rate at which hydrogen is introduced ontothe first electrode.

The structure of hydrogen gas sensors used in the present measurementwill next be described. Hydrogen gas sensors were fabricated which hadthe above-described structure (see FIG. 1) and in which a mixed solutionof NAFION® (5 wt. %), water, and an aliphatic lower alcohol was appliedto the sides of the first or second electrode in contact with theproton-conductive layer, as well as hydrogen gas sensors which had theabove-described structure and in which the mixed solution was notapplied. The resistance between the first and second electrodes wasmeasured at each of various H₂O concentrations using the thus-fabricatedhydrogen gas sensors. The measurement conditions were as follows. Thearea of the first electrode was the same as that of the secondelectrode.

Gas composition: H₂ (40%), CO₂ (15%), H₂O (10-30%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 1/min; and Resistance betweenapplied voltage (50 mV)/current. first and second electrode:

FIG. 7 is a graph describing the results of measurement 4 and showingthe relationship between H₂O concentration and resistance between thefirst and second electrodes for the case where the solution was appliedand for the case where the solution was not applied. The resistancebetween the first and second electrodes was determined from current thatflowed upon application of 50 mV.

As shown in FIG. 7, when a solution containing a predetermined polymerelectrolyte is applied onto the electrode surface, the contact areabetween the proton-conductive layer and the electrodes increases, sothat within a wide range of H₂O concentration, the resistance betweenthe first and second electrodes can be reduced to thereby improve theproton-conducting performance.

As described above, since a solution containing a predetermined polymerelectrolyte is applied onto the electrode surface to thereby render theproton-conductive performance excessive, the influences, for example, ofH₂O and CO present in a measurement gas atmosphere can be reduced,thereby enabling a more accurate measurement of hydrogen concentration.

Further, the results of measurements 2-4 demonstrate that when the flowsectional area of the diffusion-rate limiting portion and the electrodesurface of the first and second electrodes are optimized and a solutioncontaining a polymer electrolyte is applied onto the electrode surface,the proton-conductive performance is improved in both relative andabsolute terms, so that hydrogen concentration can be measured moreaccurately.

Next, the structure of another hydrogen gas sensor according to thepresent invention will be described. This second hydrogen gas sensordiffers from the hydrogen gas sensor shown in FIG. 1 in that it furtherincludes a reference electrode.

FIG. 8 is a sectional view of a main portion of the second hydrogen gassensor. In FIG. 8, elements having the same functions as those shown inFIG. 1 are denoted by the same reference numerals.

As shown in FIG. 8, in the second hydrogen gas sensor, a first electrode3 and a second electrode 4 are disposed on opposite surfaces,respectively, of a proton-conductive layer 2 such that the firstelectrode 3 and the second electrode 4 sandwich the proton-conductivelayer 2.A reference electrode 5 is formed on the surface of theproton-conductive layer 2 on which the second electrode 4 is formed.Further, an upper support 1 a and an lower support 1 b sandwich thefirst electrode 3, the second electrode 4, and the reference electrode5. A diffusion-rate limiting portion 6 is formed in the upper support 1a located above the first electrode 3. The diffusion-rate limitingportion 6 is provided between the first electrode and a measurement gasatmosphere, and the second electrode 4 is in contact with themeasurement gas atmosphere via an aperture 11 formed in the lowersupport 1 b. The reference electrode 5 is formed such that the referenceelectrode 5 is in contact with the proton-conductive layer 2 and is notexposed directly to a measurement gas atmosphere. The referenceelectrode 5 provides a reference potential.

A series circuit comprising a power source 9 and an ammeter 8 isconnected between the first and second electrodes 3 and 4 via leadportions, so that a voltage is applied between the first and secondelectrodes 3 and 4, and current flowing between the first and secondelectrodes 3 and 4 can be measured. A voltmeter 10 is connected betweenthe first electrode 3 and the reference electrode 5 via lead portions.Further, a control circuit is formed between the voltmeter 10 and thepower source 9 in order to variably control the voltage applied betweenthe first and second electrodes 3 and 4 in accordance with the potentialdifference between the first electrode 3 and the reference electrode 5.

Next, the measurement principle of the hydrogen gas sensor having areference electrode will be described with reference to FIG. 8.

(1) When hydrogen reaches the first electrode 3 via thegas-diffusion-rate limiting portion 6, an electromotive forcecorresponding to the hydrogen concentration is generated between thefirst electrode 3 and the reference electrode 5 across theproton-conductive layer 2.

(2) A control voltage is applied between the first electrode 3 and thesecond electrode 4 such that the hydrogen concentration on the firstelectrode 3 becomes constant; i.e., the potential difference between thefirst electrode 3 and the reference electrode 5 becomes constant.

(3) The hydrogen having reached the first electrode 3 via thegas-diffusion-rate limiting portion 6 is dissociated into protons byvirtue of the catalytic action of the catalytic component, such as Pt,carried on the first electrode 3 and the voltage applied between thefirst electrode 3 and the second electrode.

(4) The protons thus generated are conducted to the second electrode 4via the proton-conductive layer 2 and are converted to hydrogen on thesecond electrode 4, which hydrogen diffuses into the measurement gasatmosphere via the aperture 11. When the above-described voltage appliedin a controlled manner is sufficiently high so that a saturation currentflows between the first electrode 3 and the second electrode 4, thecurrent flowing between the first electrode 3 and the second electrode 4varies in proportion to the hydrogen concentration. Therefore, thehydrogen concentration can be measured by detecting the saturationcurrent using ammeter 8.

Since the voltage applied between the first and second electrodes 3 and4 is controlled such that the hydrogen concentration on the firstelectrode 3 is maintained constant, a high voltage can be applied whenthe hydrogen concentration of the gas under measurement is high, and lowvoltage can be applied when the hydrogen concentration of the gas undermeasurement is low. In other words, an optimal voltage can be appliedbetween the first and second electrodes 3 and 4 in accordance withhydrogen concentration. Further, in the gas sensor having the referenceelectrode (see FIG. 8), when the resistance between the first and secondelectrodes 3 and 4 increases for some reason, the applied voltageautomatically changes properly, or can be changed properly. Therefore,hydrogen concentration can be measured accurately, while the influenceof H₂O and the like is further suppressed.

Embodiment 4, Measurement 5

Next, the results of measurement performed using the hydrogen gas sensorhaving a reference electrode (see FIG. 8) will be described. In thehydrogen gas sensor, the proton-conductive layer was formed of NAFION®;the first and second electrodes and the reference electrode were porouscarbon electrodes carrying a catalyst such as Pt on the side in contactwith the proton-conductive layer; the support was formed of densealumina ceramic; and the diffusion-rate limiting portion was formed ofporous alumina ceramic.

Further, in order to stabilize the hydrogen concentration on thereference electrode, the reference electrode was employed as aself-generation reference electrode. Specifically, a constant smallcurrent was caused to flow from the first electrode to the referenceelectrode to thereby supply protons to the reference electrode, wherebya portion of hydrogen generated on the reference electrode was leaked tothe outside via a predetermined leakage resistance portion (small hole).

Moreover, as in the above-described embodiments 1 to 3, the flowsectional area of the diffusion-rate limiting portion and the electrodesurface of the first and second electrodes were optimized, and asolution containing a polymer electrolyte was applied onto the surfacesof the first and second electrodes in contact with the proton-conductivelayer, so that that the proton-conductive performance was improved inboth relative and absolute terms. Specifically, the flow sectional areaof the diffusion-rate limiting portion was set to 1.4 mm², the electrodearea of the first electrode was set to 14 mm², and the electrode area ofthe second electrode was set to 10 mm².

For each of various H₂O concentrations (H₂O concentrations of a gasunder measurement), the current flowing between the first and secondelectrodes was measured, while hydrogen concentration was varied. Themeasurement conditions were as follows.

Gas composition: H₂ (0-40%), CO₂ (15%), H₂O (10-30%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 L/min; Potential differencebetween first 150 mV; and electrode and reference electrode (controltarget): Constant current for self-generation 10 μA reference electrode:

FIG. 9 is a graph describing the results of measurement 5 and showingthe relationship between hydrogen concentration and current (currentflowing between the first and second electrodes) at each of differentH₂O concentrations. FIG. 10 shows the voltage (control voltage appliedbetween the first and second electrodes) at that time.

As shown in FIG. 9, current curves for different H₂O concentrationsbecome substantially identical over a wide range of hydrogenconcentration. This indicates that when a reference electrode is usedand the rate of proton conduction between the first and secondelectrodes is render sufficiently high, accurate measurement of hydrogenconcentration becomes possible, while the influence of H₂O, etc. isreduced.

As shown in FIG. 10, the voltage applied between the first and secondelectrodes varies with hydrogen concentration, and the applied voltageincreases as H₂O concentration decreases. Therefore, provision of thereference electrode enables application of an optimal voltage betweenthe first and second electrodes even when the conditions of ameasurement gas atmosphere; e.g., gas composition, varies.

Embodiment 5, Measurement 6

Next the results of measurement will be described in which the gaspressure dependency of the hydrogen gas sensor having a referenceelectrode (see FIG. 8) was measured by use of hydrogen gas sensors whosediffusion-rate limiting portions had different pore diameters (openingdiameters). Since Embodiment 5 is the same as Embodiment 4 exceptingpoints which will be described in detail below, a repeated descriptionwill be omitted.

The diffusion-rate limiting portion of each sensor was constituted of aporous alumina ceramic having an average pore diameter of 0.31 μm or 1μm or a dense alumina ceramic having through-holes each having anopening diameter of 30 μm, 50 μm, or 70 μm. The pore diameter and theopening diameter were measured through observation using a scanningelectron microscope (SEM). A measurement gas was supplied under adifferent gas pressure to each of the hydrogen gas sensors whosediffusion-rate limiting portions had different pore or openingdiameters, and the current flowing between the first and secondelectrodes was measured. Since the absolute value of the current varieddepending on the pore diameter (opening diameter) of the diffusion-ratelimiting portion, a ratio of the current at a pressure of 2.5 atm tothat at a pressure of 1 atm was measured as a current ratio. The smallerthe current ratio, the smaller the evaluated gas pressure dependency.The measurement conditions were as follows.

Gas composition: H₂ (40%), CO₂ (15%), H₂O (15%), N₂ (bal.); Gastemperature: 80° C.; Gas flow rate: 4 L/min; Gas pressure: 1 atm, 2.5atm Potential difference 150 mV; and between first electrode andreference electrode (control target): Constant current for 10 μAself-generation reference electrode:

FIG. 11 is a graph provided for describing results of measurement 6 andshowing the current ratio (relative ratio of current flowing between thefirst and second electrodes) obtained by changing the gas pressure. Asis understood from FIG. 11, the gas pressure dependency decreasesgreatly when the pore diameter (opening diameter) of the diffusion-ratelimiting portion is 1 μm or greater.

The present invention provides a hydrogen gas sensor capable ofaccurately measuring hydrogen concentration in the presence of a varietyof interfering gasses. Further, use of the hydrogen gas sensor of thepresent invention enables accurate measurement of hydrogen concentrationof a fuel gas used for polymer electrolyte fuel cells.

Having described specific preferred embodiments of the presentinvention, it is to be understood that the invention is not limited tothose precise embodiments, and that various changes and modificationsmay be effected therein by one skilled in the art without departing fromthe scope of the invention as defined in the appended claims.

This application is based on Japanese Patent Application No. Hei.11-333422 filed Nov. 24, 1999 which is incorporated herein by referencein its entirety.

1. A hydrogen gas sensor comprising: a proton-conductive layer formed ofa polymer electrolyte; first and second electrodes provided in contactwith the proton-conductive layer; a diffusion-rate limiting portiondisposed between the first electrode and an atmosphere of a gas undermeasurement containing hydrogen; and a circuit for applying a voltagebetween the first and second electrodes such that hydrogen introducedfrom the atmosphere via the diffusion-rate limiting portion undergoesdissociation, decomposition, or reaction to produce protons on the firstelectrode, and for determining the hydrogen concentration of the gasunder measurement based on a saturation current which flows as a resultof conduction of protons from the first electrode to the secondelectrode via the proton-conductive layer; said sensor having aproton-conducting rate from the first electrode to the second electrodethat is greater than a rate at which protons derived from hydrogen areintroduced onto the first electrode via the diffusion-rate limitingportion, and wherein the gas-diffusion resistance of the diffusion-ratelimiting portion is set such that current (a)>current (b): current (a)is a current flowing between the first and second electrodes uponapplication of a voltage of 50 mV or higher between the first and secondelectrodes in a state in which the gas-diffusion resistance of thediffusion-rate limiting portion is 0.9 mA/mm² or more with currentconversion at H₂=40% and the measurement gas has a H₂O concentration of10% or less at 80° C. or a CO concentration of 1,000 ppm or greater; andcurrent (b) is a saturation current flowing between the first and secondelectrodes in a state in which the gas-diffusion resistance of thediffusion-rate limiting portion is less than 0.9 mA/mm² with currentconversion at H₂=40% and the measurement gas has a H₂O concentration of15% or greater at 80° C. or a CO concentration of 800 ppm or less. 2.The hydrogen gas sensor as claimed in claim 1, wherein thediffusion-rate limiting portion comprises a dense body having athrough-hole having an opening diameter of 1 μm or higher.
 3. Thehydrogen gas sensor as claimed in claim 2, wherein the opening diameterof the trough-hole is 30 μm or higher.
 4. The hydrogen gas sensor asclaimed in claim 2, wherein the opening diameter of the through-hole is1 μm or higher and 70 μm or lower.
 5. A hydrogen gas sensor comprising:a proton-conductive layer formed of a polymer electrolyte; first andsecond electrodes and a reference electrode provided in contact with theproton-conductive layer; a diffusion-rate limiting portion disposedbetween the first electrode and an atmosphere of a gas under measurementcontaining hydrogen; and a circuit for applying a voltage between thefirst and second electrodes such that a constant voltage developsbetween the first electrode and the reference electrode, and such thathydrogen gas introduced from the atmosphere via the diffusion-ratelimiting portion undergoes dissociation, decomposition, or reaction toproduce portions on the first or second electrode, and for detecting thehydrogen concentration of the gas under measurement based on asaturation current which flows as a result of conduction of protons viathe proton-conductive layer; wherein said sensor having a protonconducting rate from the first electrode to the second electrode that isgreater than a rate at which protons derived from hydrogen areintroduced onto the first electrode via the diffusion-rate limitingportion, and wherein the gas-diffusion resistance of the diffusion-ratelimiting portion is set such that current (a)>current (b): current (a)is a current flowing between the first and second electrodes uponapplication of a voltage of 50 mV or higher between the first and secondelectrodes in a state in which the gas-diffusion resistance of thediffusion-rate limiting portion is 0.9 mA/mm² or more with currentconversion at H₂=40% and the measurement gas has a H₂O concentration of10% or less at 80° C. or a CO concentration of 1,000 ppm or greater;current (b) is a saturation current flowing between the first and secondelectrodes in a state in which the gas-diffusion resistance of thediffusion-rate limiting portion is less than 0.9 mA/mm² with currentconversion at H₂=40% and the measurement gas has a H₂O concentration of15% or greater at 80° C. or a CO concentration of 800 ppm or less. 6.The hydrogen gas sensor as claimed in claim 5, wherein thediffusion-rate limiting portion comprises a dense body having athrough-hole having an opening diameter of 1 μm or higher.
 7. Thehydrogen gas sensor as claimed in claim 6, wherein the opening diameterof the through-hole is 30 μm or higher.
 8. The hydrogen gas sensor asclaimed in claim 6, wherein the opening diameter of the through-hole is1 μm or higher and 70 μm or lower.